Ever since time division switching was introduced, designs have been produced for large capacity switches and network organizations. For example, a basic switch with a capacity of 32 PCM links is described in CIT-ALCATEL's French Pat. No. 1511678.
Multistage switching networks are described in French Pat. Nos. 6916790 (corresponding to U.S. Pat. No. 3,754,100), 7033980 (corresponding to U.S. Pat. No. 3,999,162), and 7104655. Efforts have been made to improve component modularity, to optimize the equipment installed to facilitate extensions and to increase reliability so as to reduce the number of emergency service calls. These aims have been achieved by improving the internal structure of an exchange, see for example French Pat. Nos. 6916790 and 7830715 (corresponding to U.S. Pat. No. 4,320,501), and by improving organization concerning "inter-aid" (i.e. switching over to spare units) while using as little redundant equipment as possible, see for example French Pat. No. 7818221 (corresponding to U.S. Pat. No. 4,276,637).
Further, the flexibility of digital switching has led to the functions of the switching network being extended. Thus, PCM links to signalling sender/receivers and control links between control units and terminal units, eg. units including a logic circuit for controlling telephone equipment such as signal generators, operator positions, alarm equipment, or other circuits, are now all performed via the switching network. Reference can be made, for example, to CIT-ALCATEL's French Pat. Nos. 7905971 (corresponding to U.S. Pat. No. 4,305,148) and 7911593.
The integration of computing facilities into telecommunications networks is calling for ever higher degrees of equipment modularity and reliability, together with the possibility of reconfiguring the network without manual intervention, and monitoring it as exhaustively as possible while using few specialized monitoring means in comparison with the means that would be required for implementing such methods in a centrally controlled exchange, as described for example in CIT-ALCATEL's French Pat. No. 7925475 (corresponding to U.S. Pat. No. 4,402,074).
Preferred embodiments of the present invention provide a switching network that is simple, modular, and self-checking. Its simplicity makes it possible to provide a high degree of redundancy at low cost, thereby increasing the reliability of the network and so reducing emergency service calls to the extremely rare cases of multiple faults occurring on an entire set of redundant units.